This invention relates to a sound output apparatus for an automotive vehicle which generates various acoustic or sound outputs, such as warning sounds, alarms, guide voice or speech, and others.
Conventionally, an automotive vehicle is equipped with an acoustic or sound device for generating various acoustic or sound outputs to transmit necessary information to a driver or passengers in a passenger compartment. For example, various sensors are installed on an automotive vehicle to detect dangerous or abnormal conditions of the vehicle. When these sensors detect any dangerous or abnormal conditions of the vehicle, warning sounds, alarms, and voice or speech guidance from a navigation system will be generated to evoke driver's or passenger's caution and to let the driver or passengers act or behave appropriately to avoid the encountering danger or abnormality.
However, even if warning sounds or alarms are generated, there will be the possibility that a driver or passenger cannot understand the contents or meaning of the immediately generating warning sounds or alarms and also cannot know the direction to which the driver's or passenger's caution should be directed. Hence, the action or behavior of the driver or passengers to avoid the encountering danger or abnormality will be delayed. For example, supersonic sonars may be installed at four corners of a vehicle body to detect approach of any obstacle. However, if the warning sounds or alarms notifying the approach of any obstacle are always the same, a driver or passengers will not be able to immediately know or perceive the direction from which the detected obstacle is approaching. The driver and passengers will not be able to promptly and properly respond to avoid the coming danger or increasing abnormality.
Japanese Patent Application Laid-open No. 2002-133596, in paragraph 0012 and FIG. 2, discloses a warning apparatus which produces a virtual sound source by using loudspeakers installed in a passenger compartment and changes the position of the virtual sound source or tone quality in accordance with time variation or spatial or positional change of a distance between oneself and the car ahead or behind, thereby letting a driver sense the position of other vehicle.
According to this warning apparatus, production of the virtual sound source is realized by using a multichannel surround system consisting of four channel loudspeakers disposed at four corners, i.e., right and left edges of respective front and rear ends, in a passenger compartment, or by using a stereo dipole system consisting of a pair of loudspeakers disposed adjacently to each other which is preferably incorporated in a car navigation system.
Compared with other systems, the stereo dipole system has a very wide audible area in which clear acoustic or sound image localization is attained. Furthermore, as two loudspeakers are disposed adjacently to each other, the stereo dipole system can be preferable employed as an acoustic equipment installed in a narrow or limited space available in a passenger compartment.
More specifically, to obtain clear acoustic or sound image localization according to the stereo dipole system, as shown in FIG. 13, a pair of loudspeakers SP is disposed in front of an audience with an angle θ formed between two straight lines connecting respective loudspeakers SP to the audience. In this case, the required angle θ is approximately 10°. In practice, compact loudspeakers having practically usable frequency characteristics are approximately 10 cm in diameter. If these compact loudspeakers are disposed as closely as possible, the closest distance between the centers of two loudspeakers SP will be approximately 10 cm (i.e., W=10 cm). Considering this relationship, the distance L from the audience to a plane on which sound output surfaces of two loudspeakers SP are aligned should be approximately 60 cm. If the size of loudspeakers is more reduced in the future, it may be necessary to reduce the distance L.
However, the loudspeakers incorporated in a car navigation system are usually positioned in a central region on an instrument panel or a dashboard substantially equally distant from each of a driver's seat and the next passenger's seat. In other words, the loudspeakers of a car navigation system are not positioned in direct front of the driver's seat or in direct front of the passenger's seat. Therefore, it is difficult to obtain clear acoustic or sound image localization by using a car navigation system even if the stereo dipole system is employed. If the loudspeakers are downsized in the future, the above distance L may be too long to obtain clear acoustic or sound image localization.
Furthermore, according to the stereo dipole system, it is generally known that clear acoustic or sound image localization is obtained in a wide angular range of ±70°˜90° with respect to a line (=0°) directing the front side of the audience, as shown in FIG. 13. In other words, there is an insensible angular zone at the back side of the audience in which no acoustic or sound image localization can be obtained. For example, according to the warning apparatus disclosed in the above-described prior art reference, a driver cannot obtain clear acoustic or sound image localization in an insensible region spreading at the back side of the driver when the loudspeakers of a car navigation system are positioned in front on a driver's seat.